The present invention relates generally to the earth boring art. The invention more particularly relates to the control of pressure of drilling mud within a well when gas from the formations has intruded into the well and the gas is being circulated to the surface.
It is customary to provide a choke or throttling valve in a manifold connected with the annulus of the well beneath a blowout preventer. When the blowout preventer is closed about the drill string, the choke can establish and maintain a back pressure on the drilling mud that is being diverted through the manifold. This back pressure, together with the hydrostatic pressure of the drilling mud in the well, is intended to contain the pressured fluids within formations penetrated by the well bore, i.e., prevent them from flowing into the well bore. The choke is preferably adjustable so that, in the case of a "kick" or gas bubble, it may be regulated in an attempt to maintain a predetermined pressure differential between the bottom hole pressure of the mud and the pressure of the formation fluid as heavier mud is circulated down the drill string carrying the gas bubble up the annulus to "kill" the well. During this time, it is desirable not only to contain such fluid, but also to avoid excessive back pressure in the well which might cause the drill string to stick or damage the formations, the well casing, or the well head equipment. A significant problem that is encountered in such an operation is the time delay between the adjustment of the throttling valve at the manifold and the pressure change detected at the top of the drill pipe. The pressure change produced by the adjustment of the throttling valve must traverse the casing annulus to the bottom of the well and the interior of the drill pipe to the surface.